This application relates generally to disc drives and more particularly to phase-advanced filtering of mechanical resonance.
Disc drives are data storage devices that store digital data in magnetic form on a rotating storage medium on a disc. Modern disc drives comprise one or more discs that are coated with a magnetizable medium and mounted on the hub of a spindle motor for rotation at a constant high speed. Information is stored on the discs in a plurality of concentric circular tracks typically by an array of transducers (xe2x80x9cheadsxe2x80x9d) mounted to an actuator for radial movement of the heads relative to the discs. Each of the concentric tracks is generally divided into a plurality of separately addressable data sectors. The read/write transducer, e.g. a magnetoresistive read/inductive write head, is used to transfer data between a desired track and an external environment. During a write operation, data is written onto the disc track and during a read operation the head senses the data previously written on the disc track and transfers the information to the external environment. Critical to both of these operations is the accurate locating of the head over the center of the desired track.
A problem in disc drives that limits drive performance in general and head position accuracy specifically is component vibration or resonance. Components in the disc drive exhibit resonance modes that adversely affect the performance of disc drive components. For example, because of resonance in the actuator arm, the transducer heads may not be directly over the desired tracks indicated by the servo control of the disc drive. This problem is exacerbated by the recent push to increase the tracks-per-inch (TPI) on the disc surfaces. When TPI is increased, the room for margin in head placement becomes disproportionately smaller, and servo positioning errors more frequent.
Unfortunately, component resonance cannot be completely eliminated without extreme cost. Traditional approaches to the problem of component resonance have involved utilizing a filter to compensate for the component resonances. Typically, a filter is implemented in the signal path having filter parameters such as pass band and attenuation parameters associated with a known resonance mode of a component receiving the signal. The typical filter that is used is a notch filter exhibiting an attenuation band in a generally high frequency associated with the resonance mode of the component. However, every component in a disc drive exhibits resonance modes at different frequencies. Furthermore, the resonant frequency of a given component will vary with temperature. Thus, fine-tuning a notch filter to accommodate vibrations at these many frequencies is difficult, if not impossible. Traditional approaches have included widening the attenuation band in the notch filter to attenuate a wider range of frequencies. Unfortunately, as is well known, this approach leads to significant phase margin loss as the attenuation band is widened. Phase margin loss generally means phase loss occurring at the open loop gain crossover frequency (i.e., the frequency where the open loop gain is 0 decibels (dB)).
Further aggravating the problem, is the use of Mask Read Only Memory (ROM) for disc drive boot software. Mask ROM is a one-time burn-in ROM that is typically a lower cost than other types of ROM such as flash EPROM and is frequently used to store disc drive boot code. Disc drive manufacturers typically provide an integrated circuit (IC) vendor with boot code and the IC vendor burns the boot code into a Mask ROM. The boot code in Mask ROM is run on power up to perform basic initial power up operations of the disc drive servo controller, including seeks and track following to load code from the disc. During power up, reserve tracks are typically accessed to gather or load data and/or executable code stored on the reserve tracks. To precisely position the head over the reserve tracks, a filter is employed to filter out the resonance modes previously discussed. Filter parameters are stored in Mask ROM along with the boot code.
Due to time-to-market demands, sufficient time must be given to the IC vendor to burn the boot code into the Mask ROM. Typically, disc drive manufacturers provide the IC vendor with the boot code three to four months prior to final testing and manufacture of the disc drive. Between the time that the boot code is given to the IC vendor and the time for manufacture, changes are frequently made to components in the disc drive. Changes to components result in changes in resonance modes associated with those components. Thus, the filter parameters included in the boot code given to the IC vendor may not, and frequently do not, exactly correspond to the resonance modes of components that are ultimately used in the disc drive. Consequently, notch filters in boot code often do not adequately attenuate resonant frequencies. One suggested solution is to choose filter parameters in the boot code such that the notch filter or notch filters exhibit a wide attenuation band. However, as discussed above, a wider attenuation band leads to significant phase margin loss during disc drive operation. It is well known in the art that phase margin loss leads to deterioration in disc drive performance, including, but not limited to, increased run-out and reduced servo controller bandwidth.
Accordingly, there is a need for a method and system for effectively attenuating unwanted component resonance in a disc drive, while limiting or avoiding phase margin loss.
Against this backdrop the present invention has been developed. An embodiment of the present invention is a unique system for attenuating selected resonance modes due to component vibration in a disc drive. More specifically, an embodiment is a phase-advanced filter filtering a selected frequency corresponding to a resonance mode. Still further, an embodiment is a unique low-pass filter providing significant gain attenuation at selected resonance modes, while preventing significant phase margin loss.
Embodiments of the invention may be implemented as a computer process, a computing system or as an article of manufacture such as a computer program product or computer readable media. The computer program product may be a computer storage media readable by a computer system and encoding a computer program of instructions for executing a computer process. The computer program product may also be a propagated signal on a carrier readable by a computing system and encoding a computer program of instructions for executing a computer process.
These and various other features as well as advantages which characterize embodiments of the present invention will be apparent from a reading of the following detailed description and a review of the associated drawings.